System for purging contaminants from a vacuum assisted image conditioning roll

ABSTRACT

A system for removing excess carrier liquid from an electrostatic image developed with liquid developing material made up of toner particles immersed in a liquid carrier medium on an image bearing member. The system includes an absorbent contact roller for absorbing at least a portion of the liquid carrier off of the liquid image, and vacuum source coupled to the contact roller for generating both negative pressure at the surface of the roller to draw the absorbed liquid through the contact roller, and positive air pressure for pushing contaminated liquid out of the roller.

This invention relates generally to a system for enhancing vacuumefficiency and effectiveness of a centrally evacuated permeable roller,and more particularly, concerns an improved vacuum assisted imageconditioning device for removing excess liquid from a developed liquidimage in a liquid developing material based electrostatographic printingmachine.

Generally, the process of electrostatographic copying is initiated byexposing a light image of an original document onto a substantiallyuniformly charged photoreceptive member, resulting in the creation of alatent electrostatic image of the original document on thephotoreceptive member. This latent image is subsequently developed intoa visible image by a process in which developer material is depositedonto the surface of the photoreceptive member. Typically, this developermaterial comprises carrier granules having toner particles adheringtriboelectrically thereto, wherein the toner particles areelectrostatically attracted from the carrier granules to the latentimage for forming a developed powder image on the photoreceptive member.Alternatively, liquid developing materials comprising a liquid carrierhaving toner particles immersed therein have been successfully utilizedto develop electrostatic latent images, wherein the liquid developingmaterial is applied to the photoconductive surface with the tonerparticles being attracted toward the image areas of the latent image toform a developed liquid image on the photoreceptive member. Regardlessof the type of developing material employed, the toner particles of thedeveloped image are subsequently transferred from the photoreceptivemember to a copy substrate, either directly or by way of an intermediatetransfer member. Thereafter, the image may be permanently affixed to thecopy substrate for providing a "hard copy" reproduction or print of theoriginal document or file. In a final step, the photoreceptive member iscleaned to remove any charge and/or residual developing material fromthe photoconductive surface in preparation for subsequent imagingcycles.

The above described electrostatographic reproduction process is wellknown and is useful for light lens copying from an original as well asfor printing applications involving electronically generated or storedoriginals. Analogous processes also exist in other printing applicationssuch as, for example, digital laser printing where a latent image isformed on the photoconductive surface via a modulated laser beam, orionographic printing and reproduction where charge is deposited on acharge retentive surface in response to electronically generated orstored images. Some of these printing processes develop toner on thedischarged area, known as DAD, or "write black" systems, asdistinguished from so-called light lens generated image systems whichdevelop toner on the charged areas, also known as CAD, or "write white"systems. The subject invention applies to both such systems.

The use of liquid developer materials in imaging processes is wellknown. Likewise, the art of developing electrostatographic latent imagesformed on a photoconductive surface with liquid developer materials isalso well known. Indeed, various types of liquid developing materialsand liquid based development systems have heretofore been disclosed withrespect to electrostatographic printing machines. Liquid developers havemany advantages, and often produce images of higher quality than imagesformed with dry developing materials. For example, the toner particlesutilized in liquid developing materials can be made to be very smallwithout the resultant problems typically associated with small particlepowder toners, such as airborne contamination which can adversely affectmachine reliability and can create potential health hazards. The use ofvery small toner particles is particularly advantageous in multicolorprocesses wherein multiple layers of toner generate the final multicoloroutput image. Further, full color prints made with liquid developers canbe processed to a substantially uniform finish, whereas uniformity offinish is difficult to achieve with powder toners due to variations inthe toner pile height as well as a need for thermal fusion, among otherfactors. Full color imaging with liquid developers is also economicallyattractive, particularly if surplus liquid carrier containing the tonerparticles can be economically recovered without cross contamination ofcolorants.

Liquid developer material typically contains about 2 percent by weightof fine solid particulate toner material dispersed in the liquidcarrier, typically a hydrocarbon. After development of the latent image,the developed image on the photoreceptor may contain about 12 percent byweight of the particulate toner in the liquid hydrocarbon carrier.However, at this percent by weight of toner particles, developed liquidimages tend to exhibit poor cohesive behavior which results in imagesmear during transfer. In addition, partial image removal, or so-calledscavenging, is problematic during successive liquid development steps,particularly in image-on-image color processes. In order to preventimage scavenging and to improve the quality of transfer of the developedimage to a copy sheet, the liquid developing material making up thedeveloped liquid image is typically "conditioned" by compressing orcompacting the toner particles in the developed image and removingcarrier liquid therefrom for increasing the toner solids contentthereof. This can be accomplished by either: conditioning the liquid inkmaking up the image into the image areas so as to physically stabilizethe image on the photoreceptor or other image bearing surface; byconditioning liquid ink placed on the surface of the photoreceptor orother image bearing surface prior to the point where the image isdeveloped with the liquid ink; or by conditioning the liquid ink streamas the ink is being delivered to the image bearing surface. Liquid inkconditioning greatly improves the ability of the toner particles to forma high resolution image on the final support substrate or anintermediate transfer member, if one is employed.

Various devices and systems are known for effectively conditioningliquid developing materials in electrostatographic systems. In oneexemplary system particularly relevant to the present invention, adevice and method for increasing the solid content of an image formedfrom a liquid developer is provided, wherein an absorbtive blottingmaterial is contacted with the developed liquid image. A vacuum sourceis coupled to the blotting material so that absorbed liquid dispersantis drawn through the blotting material. The absorptive blotting materialis preferably provided in the form of a covering on a porous conductiveroller which is biased with an electrical charge having a polarity whichis the same as the charge of the toner particles in the developingmaterial, such that the resulting electric field repels the tonerparticles from the absorptive blotting material for transferring so thatminimal toner particles thereto. The roller defines a central cavity towhich the vacuum is coupled, forming a centrally evcuated permeableroller system.

Several advantages have been found in eliminating excess liquid carrierby vacuuming the liquid through a roller member, a belt, or othercontact member. For example, in a vacuum assisted system, lessdispersant evaporates into the atmosphere, thereby reducing pollutionand potential health risks to individuals working near the machine. Inaddition, since the liquid carrier can be reclaimed and reused, anefficient vacuum assisted blotter roller can yield cost advantages.Furthermore, the use of a vacuum assisted system may eliminate thepotential for removed liquid to return back to the image bearing surfacefrom the contact member, thereby eliminating potential disturbance ofthe image such that the final output image tends to be more clearlydefined.

Although various systems have been developed for conditioning an imagein liquid based electrostatographic printing systems, some problems andinadequacies remain with respect to known electrostatically basedsystems. In particular, notwithstanding the use of electrical fields torepel toner particles from the absorption material, some tonerparticles, as well as other contaminants, such as paper debris and thelike, may make their way into the absorption material. Thus, thesesystems tend to have a limited operational life due to clogging of poreswithin the absorption material, either by toner particles orcontaminants which collect in the system.

The present invention is directed toward a system for enhancing vacuumefficiency and reducing contaminant entrapment in a vacuum assistedliquid removal system wherein a centrally evacuated permeable rollersystem is used to extract liquid from a wetted surface. Morespecifically, with respect to the field of liquid developingmaterial-based electrostatographic copying and printing, the presentinvention is directed toward an electrostatic image conditioning devicein which image compaction and liquid removal is accomplished via aporous roll member with the assistance of a vacuum system coupledthereto. In the particular invention disclosed herein, negative airpressure is generated within the porous roll member for drawing liquidcarrier away from a developed liquid image on a photoreceptor or otherimage bearing surface, while positive air pressure is also generated forpushing liquid carrier and/or air through the porous roll member topurge the roll of contaminants therein. For purposes of the presentdiscussion, negative air pressure will be defined as pressure betweenzero and the surrounding ambient environment, usually taken to be oneatmosphere while positive air pressure will be defined as pressuregreater than the surrounding ambient environment or greater than oneatmosphere. The following disclosures may be relevant to some aspects ofthe present invention:

U.S. Pat. No. 4,286, 039 Patentee: Landa et al. Issued: Aug. 25, 1981U.S. Pat. No. 5,332,642 Patentee: Simms et al. Issued: Jul. 26, 1994U.S. Pat. No. 5,493,369 Patentee: Sypula et al. Issued: Feb. 20, 1996

The relevant portions of the foregoing patents may be briefly summarizedas follows:

U.S. Pat. No. 4,286,039 discloses an image forming apparatus comprisinga deformable polyurethane roller, which may be a squeegee roller orblotting roller which is biased by a potential having a sign the same asthe sign of the charged toner particles in a liquid developer. The biason the polyurethane roller is such that it prevents streaking, smearing,tailing or distortion of the developed electrostatic image and removesmuch of the liquid carrier of the liquid developer from the surface ofthe photoconductor.

U.S. Pat. No. 5,332,642 discloses a porous roller for increasing thesolids content of an image formed from a liquid developer. The liquiddispersant absorbed through the roller is vacuumed out through a centralcavity of the roller. The roller core and/or the absorbent materialformed around the core may be biased with the same charge as the tonerso that the toner is repelled from the roller while the dispersant isabsorbed.

U.S. Pat. No. 5,493,369 discloses a roller for improved conditioning ofan image formed from a liquid developer comprised of toner particles andliquid carrier. A wire mesh uniformly covering an inner layer of theroller uniformly distributes an electrical bias closer to the surface ofthe roller and to the adjacent image bearing surface. The electricalbias has the same sign polarity as that of the toner particles of theimage for electrostatically repelling the toner particles and preventingthe toner particles from entering the roller, and for compacting thetoner particles to the image. The wire mesh reduces the electricalrequirements of the materials used for the roller.

In addition to the above cited references, it is noted that varioustechniques have been devised for removing excess liquid carrier from animaging member which may involve a vacuum removal system and/or anelectrical bias applied to a portion of the liquid dispersant removaldevice. The following additional references may be relevant:

U.S. Pat. No. 4,878,090 discloses a development apparatus comprising avacuum source which draws air around a shroud to remove excess liquidcarrier from the development zone.

U.S. Pat. No. 5,023,665 discloses an excess liquid carrier removalapparatus for an electrophotographic machine. The apparatus is comprisedof an electrically biased electrode having a slit therein coupled to avacuum pump. The vacuum pump removes, through the slit in the electrode,liquid carrier from the space between the electrode and thephotoconductive member. The electrical bias generates an electricalfield so that the toner particle image remains undisturbed as the vacuumwithdraws air and liquid carrier from the gap.

U.S. Pat. No. 5,481,341 having a common assignee as the presentapplication, discloses a belt used for absorbing liquid toner dispersantfrom a dispersant laden image on a electrostatographic imaging member orintermediate transfer member. The angle of contact of the absorptionbelt is adjusted with respect to the image bearing member formaintaining proper cohesiveness of the image and absorption of liquiddispersant. The absorption belt is passed over a roller biased with thesame charge as the toner. A pressure roller is in contact with theabsorption belt for removal of liquid therefrom.

U.S. Pat. No. 5,424,813, having a common assignee as the presentapplication, discloses a roller comprising an absorption material and acovering, which are adapted to absorb liquid carrier from a liquiddeveloper image. The covering has a smooth surface with a plurality ofperforations, to permit liquid carrier to pass through to the absorptionmaterial at an increased rate, while maintaining a covering having asmooth surface which is substantially impervious to toner particles yetpervious to liquid carrier so as to inhibit toner particles fromdeparting the image.

U.S. Pat. No. 5,481,341, having a common assignee as the presentapplication, discloses a roller for controlling application of carrierliquid to form a liquid developed image, comprising a rigid porouselectroconductive supportive core, a conformable microporous resistivefoam material provided around the core, and a pressure controller forproviding a positive or negative load pressure to the roller.

In accordance with one aspect of the present invention, there isprovided a system for removing excess liquid from a liquid developedimage having toner particles immersed in a liquid carrier on an imagebearing surface, comprising: an absorbent contact member for contactingthe liquid developed image on the image bearing member to absorb atleast a portion of the liquid carrier therefrom; and a bi-directionalvacuum system coupled to the absorbent contact member for selectivelygenerating a negative pressure airflow through the absorbant contactmember so as to draw absorbed liquid carrier therethrough and a positivepressure airflow through the absorbant contact member so as to pushabsorbed liquid carrier and residual contaminants from the absorbentcontact member.

In accordance with another aspect of the present invention, a liquid inktype electrostatographic printing machine is provided, including asystem for removing excess liquid from a liquid developed image havingtoner particles immersed in a liquid carrier on an image bearingsurface, comprising: an absorbent contact member for contacting theliquid developed image on the image bearing member to absorb at least aportion of the liquid carrier therefrom; and a bi-directional vacuumsystem coupled to the absorbent contact member for selectivelygenerating a negative pressure airflow through the absorbant contactmember so as to draw absorbed liquid carrier therethrough and a positivepressure airflow through the absorbant contact member so as to pushabsorbed liquid carrier and residual contaminants from the absorbentcontact member.

In accordance with yet another aspect of the invention, there isprovided an improved vacuum assisted permeable roller system for removalof liquid from a wetted surface, comprising: a rigid porous core memberand an absorbent contact layer for contacting the wetted surface toabsorb at least a portion of the liquid therefrom; and a bidirectionalvacuum system coupled to the permeable roller member for selectivelygenerating a negative pressure airflow through the absorbant contactlayer so as to draw absorbed liquid therethrough and a positive pressureairflow through the absorbant contact layer so as to push absorbedliquid and residual contaminants from the absorbent contact layer.

Other aspects of the present invention will become apparent as thefollowing description proceeds and upon reference to the drawings, inwhich:

FIG. 1 is a schematic elevational view of an exemplary embodiment of avacuum assisted image conditioning system for removing excess liquidfrom liquid developed images in accordance with the present invention;

FIG. 2 is a schematic elevational view of an alternative embodiment fora vacuum assised, image conditioning system in accordance with thepresent invention; and

FIG. 3 is a schematic, elevational view of a liquid ink-basedimage-on-image color electrostatographic printing machine incorporatinga vacuum assisted image conditioning system in accordance with thepresent invention.

For a general understanding of the features of the present invention,reference is made to the drawings, wherein like reference numerals havebeen used throughout to designate identical elements. FIG. 3 shows aschematic elevational view of a full-color, liquid developing materialbased electrostatographic printing machine incorporating the features ofthe present invention. Inasmuch as the art of electrostatographicprinting is well known, the various processing stations employed in theprinting machine of FIG. 2 will be described only briefly with referencethereto while the present description will focus on a detaileddescription of the particular features vacuum assisted imageconditioning system of the present invention. It will become apparentfrom the following discussion that the apparatus of the presentinvention may also be well-suited for use in a wide variety of systems,devices, apparatus and machines and is not necessarily limited in itsapplication to the field of electrostatographic printing or theparticular liquid developing material-based electrostatographic machinedescribed herein. As such, it will be understood that the presentlydescribed system and method provided by this invention, is not limitedto use in printing engines but is capable of providing enhanced liquidremoval from any wetted surface. As such, while the present inventionwill hereinafter be described in connection with preferred embodimentsthereof, it will be understood that the description of the invention isnot intended to limit the invention to this preferred embodiment. On thecontrary, the description is intended to cover all alternatives,modifications, and equivalents as may be included within the spirit andscope of the invention as defined by the appended claims.

Turning now to FIG. 3, the multicolor electrostatographic printingmachine shown employs a photoreceptive belt 10 which is transported inthe direction of arrow 16, along a curvilinear path defined by rollers12 and 14. These rollers are driven in the direction of arrows 13 foradvancing successive portions of the photoreceptive belt 10 sequentiallythrough the various processing stations disposed about the path ofmovement thereof. Initially, the belt 10 passes through a chargingstation where a corona generating device 20 charges the photoconductivesurface of belt 10 to relatively high, substantially uniform electricalpotential.

After the substantially uniform charge is placed on the photoreceptivesurface of the belt 10, the printing process proceeds by either placingan input document from a transparent imaging platen (not shown), or byproviding a computer generated image signal for discharging thephotoconductive surface in accordance with the image information to begenerated. The present description is directed toward a Recharge,Expose, and Develop (REaD) color imaging process, wherein the chargedphotoconductive surface of photoreceptive member 10 is serially exposedby a series of individual raster output scanners (ROSs) 22, 32, 42 torecord a series of latent images thereon. The photoconductive surface iscontinuously recharged and re-exposed to record latent images thereoncorresponding to the subtractive primary of another color of theoriginal. Each latent image is serially developed with appropriatelycolored toner particles until all the different color toner layers aredeposited in superimposed registration with one another on thephotoconductive surface. It will be recognized that this REaD processrepresents only one of various multicolor processing techniques that maybe used in conjunction with the present invention, and that the presentinvention is not intended to be limited to REaD processing or tomulticolor processes.

In the exemplary electrostatographic system of FIG. 2, each of the colorseparated electrostatic latent images are serially developed on thephotoreceptive belt 10 via a fountain-type developing apparatus 24, 34,44 and 54, which may be of the type disclosed, for example in U.S. Pat.No. 5,579,473, wherein appropriately colored developing material istransported into contact with the surface of belt 10. Each differentcolor developing material is comprised of charged toner particlesdisseminated through the liquid carrier, wherein the toner particles areattracted to the latent image areas on the surface of belt 10 byelectrophoresis for producing a visible developed image thereon.Generally, in a liquid developing material-based system, the liquidcarrier medium makes up a large amount of the liquid developingcomposition. Specifically, the liquid medium is usually present in anamount of from about 80 to about 98 percent by weight, although thisamount may vary from this range.

By way of example, the liquid carrier medium may be selected from a widevariety of materials, including, but not limited to, any of severalhydrocarbon liquids, such as high purity alkanes, including Norpar® 12,Norpar® 13, and Norpar® 15, and including isoparaffinic hydrocarbonssuch as Isopar® G, H, L, and M, available from Exxon Corporation. Otherexamples of materials suitable for use as a liquid carrier includeAmsco® 460 Solvent, Amsco® OMS, available from American Mineral SpiritsCompany, Soltrol®, available from Phillips Petroleum Company, Pagasol®,available from Mobil Oil Corporation, Shellsol®, available from ShellOil Company, and the like. Isoparaffinic hydrocarbons provide apreferred liquid media, since they are colorless, environmentally safe,and possess a sufficiently high vapor pressure so that a thin film ofthe liquid evaporates from the contacting surface within seconds atambient temperatures. The toner particles can be any pigmented particlecompatible with the liquid carrier medium, such as those contained inthe developing materials disclosed in, for example, U.S. Pat. Nos.3,729,419; 3,968,044; 4,476,210; 4,794,651; and 5,451,483, amongnumerous other patents. The toner particles preferably have an averageparticle diameter from about 0.2 to about 10 microns, and more preciselyfrom about 0.5 to about 2 microns. The toner particles may be present inamounts of from about 1 to about 10 percent by weight, and preferablyfrom about 1 to about 4 percent by weight of the developer composition.The toner particles can consist solely of pigmented particles, or maycomprise a resin and a pigment; a resin and a dye; or a resin, apigment, and a dye. Dyes generally are present in an amount of fromabout 5 to about 30 percent by weight of the toner particle, althoughother amounts may be present provided that the objectives of the presentinvention are achieved. Generally, any pigment material is suitableprovided that it consists of small particles and that combine well withany polymeric material also included in the developer composition.Pigment particles are generally present in amounts of from about 5 toabout 40 percent by weight of the toner particles, and preferably fromabout 10 to about 30 percent by weight. In addition to the liquidcarrier vehicle and toner particles which typically make up the liquiddeveloper materials suitable for use in a liquid developing materialbased electrostatographic machine, a charge control additive (sometimesreferred to as a charge director) may also be included for facilitatingand maintaining a uniform charge on toner particles by imparting anelectrical charge of selected polarity (positive or negative) to thetoner particles. The charge control additive may be present in an amountof from about 0.01 to about 3 percent by weight, and preferably fromabout 0.02 to about 0.05 percent by weight of the developer composition.

The developer station may also include a metering roll 25, 35, 45, 55situated adjacent to a corresponding developer fountain 24, 34, 44, 54and in close proximity to the surface of photoreceptive belt 10. Themetering roll generally rotates in a direction opposite the movement ofthe photoconductor surface so as to exert a shear force on the liquiddeveloped image in the area of the nip formed between the surface of thephotoreceptor and the metering roll. This shear force removes an initialamount of liquid developing material from the surface of thephotoreceptor so as to minimize the thickness of the developing materialthereon. The excess developing material removed by the metering rolleventually falls away from the rotating metering roll for collection ina sump, not shown. A DC power supply 26, 36, 46, 56 may also be providedfor maintaining an electrical bias on the metering roll at a selectedpolarity for enhancing image development. Each of the developer stationsshown in FIG. 2 are substantially identical to one another and representonly one of various known apparatus or systems that can be utilized toapply liquid developing material to the photoconductive surface or otherimage recording medium.

After image development, it is generally desirable that the liquiddeveloped image be processed or conditioned to compress the image and toremove additional excess liquid carrier therefrom, as shown, forexample, by U.S. Pat. Nos. 4,286,039 and 5,493,369, among various otherpatents. This so-called "image conditioning" process is directed towardincreasing the solids percentage of the image, and can advantageouslyincrease the solids percentage of the image to a range of approximately25% or higher. An exemplary apparatus for image conditioning is depictedat reference numerals 28, 38, 48 and 58, each comprising a roller memberwhich preferably includes a porous body and a perforated skin covering.In addition, the image conditioning rolls 28, 38, 48 and 58 aretypically conductive and biased to a potential having a polarity whichrepels the charged toner particles of the liquid developed image tocompress the image and to inhibit the departure of toner particlestherefrom. In an exemplary image conditioning system of U.S. Pat. No.5,332,642, incorporated by reference herein, a vacuum source (not shown)may also be provided, coupled to the interior of the roller, forcreating an airflow through the porous roller body to draw liquidcarrier from the surface of the photoreceptor 10 for enhancing theprocess of increasing the percentage of toner solids in the developedimage.

In operation, rollers 28, 38, 48 and 58 rotate in contact with theliquid image on belt 10 such that the porous body of roller 28 absorbsexcess liquid from the surface of the image through the pores andperforations of the roller skin covering. The vacuum source draws liquidthrough the roller skin to a central cavity, wherein the collectedliquid may be deposited in a receptacle or some other location whichpermits either disposal or recirculation of the liquid carrier. Theporous roller is thus continuously discharged of excess liquid toprovide constant removal of liquid from the developed image on belt 10.It will be recognized by one of skill in the art that the vacuumassisted liquid absorbing roller described hereinabove may also finduseful application in an embodiment in which the image conditioningsystem is provided in the form of a belt, whereby excess liquid carrieris absorbed through an absorbent foam layer in the belt, as described inU.S. Pat. Nos. 4,299,902 and 4,258,115.

As previously noted, the present invention is directed toward animproved vacuum assisted image conditioning device, wherein the vacuumsystem coupled to the central cavity of the porous roller comprises abidirectional vacuum source or sources for producing a vacuumarrangement that reduces the amount of contaminants residing in theroller, thereby yielding higher vacuum pressure in the critical areaadjacent the developed liquid image on the photoreceptor and enhancingthe operational life of the imaging conditioning roll. This improvedvacuum assisted image conditioning apparatus, and a bidirectional vacuumarrangement in accordance therewith, will be described in detailfollowing the present description of the electrostatographic printingprocess.

Moving on the with the discussion of illustrative multicolor printingprocess, imaging, development and image conditioning are repeated forsubsequent color separations by recharging and reexposing the belt 10via charging devices 30, 40 and 50 as well as exposure devices 32, 42and 52, whereby color image information is superimposed over theprevious developed image. For each subsequent exposure an adaptiveexposure processing system may be employed for modulating the exposurelevel of the raster output scanner (ROS) 32, 42 or 52 for a given pixelas a function of the developing material previously developed at thepixel site, thereby allowing toner layers to be made independent of eachother, as described in U.S. Pat. No. 5,477,317. The reexposed image isnext advanced through a corresponding development station andsubsequently through an associated image conditioning station, forprocessing in the manner previously described. Each step is repeated aspreviously described to create a multilayer image made up of black,yellow, magenta, and cyan toner particles as provided via eachdeveloping station. It should be evident to one skilled in the art thatthe color of toner at each development station could be provided in adifferent arrangement.

After the multilayer image is created on the photoreceptive member 10,it may be advanced to an intermediate transfer station 70 fortransferring the image from the photoconductive belt 10 to anintermediate transfer member, identified by reference numeral 80.Thereafter, the intermediate transfer member continues to advance in thedirection of arrow 82 to a transfer nip 94 where the developed image istransferred and affixed to a recording sheet 100 transported through nip94 in the direction of arrow 96. While the image on the photoreceptor10, after image conditioning thereof, and consequently the imagetransferred to the intermediate transfer member 80, has a solidspercentage in the range of approximately 25%, the optimal solids contentfor transfer of a liquid image to a copy substrate is aboveapproximately 50%. This solids percentage insures minimal hydrocarbonemissions from an image bearing copy substrate and furtheradvantageously minimizes or eliminates carrier showthrough on the copysubstrate. Thus, it is also desirable to remove excess liquid from thedeveloped image on the intermediate 80, prior to transfer of that imageto the copy sheet 100. To that end, prior to transfer of the image fromthe intermediate transfer member, the liquid developed image thereonmay, once again, be conditioned in a manner similar to the imageconditioning process described with respect to image conditioningapparatus 28, 38, 48 and 58. Thus, as shown in FIG. 2, an additionalimage conditioning apparatus 88 is provided adjacent the intermediatetransfer member 80 for conditioning the image thereon.

Thereafter, transfer of the liquid developed image from the intermediatetransfer member to the copy substrate 100 can be carried out by anysuitable technique conventionally used in electrophotography, such ascorona transfer, pressure transfer, bias roll transfer, and the like. Itwill be understood that transfer methods such as adhesive transfer, ordifferential surface energy transfer, wherein the receiving substratehas a higher surface energy with respect to the developing materialmaking up the image, can also be employed.

After the developed image is transferred to intermediate member 80,residual liquid developer material may remain on the photoconductivesurface of belt 10. A cleaning station 60 is therefore provided, whichmay include a roller formed of any appropriate synthetic resin which maybe driven in a direction opposite to the direction of movement of belt10, for scrubbing the photoconductive surface clean. It will beunderstood, however, that a number of photoconductor cleaning devicesexist in the art, any of which would be suitable for use with thepresent invention. In addition, any residual charge left on thephotoconductive surface may be extinguished by flooding thephotoconductive surface with light from a lamp (not shown) inpreparation for a subsequent successive imaging cycle. In this way,successive electrostatic latent images may be developed.

The foregoing discussion provides a general description of the operationof a liquid developing material based electrostatographic printingmachine which may advantageously incorporate the improved bi-directionalor bilateral vacuum assisted image conditioning system of the presentinvention. The detailed structure of the improved bilateral vacuumassisted image conditioning system will be described hereinafter withreference to FIG. 1.

Referring now to FIG. 1 a preferred embodiment of the the bi-directionalvacuum assisted image conditioning system for purging contaminants froma vacuum assisted image conditioning role in accordance with the presentinvention will be described, with an understanding that the imageconditioning systems shown in the multicolor electrostatographicprinting system of FIG. 3, identified by reference numerals 28, 38, 48,58 and 88, are substantially identical thereto. In general, the onlymajor distinction between each image conditioning system is the liquiddeveloped image being conditioned, with minor distinctions possiblybeing found in spacing and bias voltage levels due to developed imagepile height differences.

FIG. 1 depicts one of various embodiments for a vacuum assisted imageconditioning system in accordance with the present invention, whichgenerally includes a porous roll member 110 in the form of an absorbentcylindrical contact roller coupled to a vacuum system 120. The rollmember 110 is positioned adjacent to an image bearing surface 10 whichtransports a developed liquid image 102 into contact therewith forremoving at least a portion of the liquid carrier from the liquid image102. A high voltage bias supply 130 may also be provided, for biasingthe roll member 110 to the same polarity as that of the toner particlesin the developed liquid image so that the toner particles areelectrostatically repelled away from the surface of the roll member 110.This electrical bias may also act to electrostatically compact the imageon the image bearing 10, enabling physical stabilization of the tonerparticles within the developed liquid image area. One exemplary vacuumassisted porous roller system known in the art which may be effectivelyused to condition an image formed of a liquid developing material isgenerally disclosed in commonly assigned U.S. Pat. No. 5,332,642,previously incorporated herein by reference, wherein a negative pressurevacuum system is coupled to an absorbent blotter roller to draw offliquid carrier dispersant through the absorbent material which, in turn,removes excess carrier liquid from the developed liquid image.

Describing this vacuum assisted liquid removal apparatus in greaterdetail, roll member 110 is generally comprised of a rigid porous supportcore 114 which may be in the form of a cylindrical tube defining ahollow central cavity extending along the entire length of the roller110. A conformable, preferably microporous, absorbent material, whichmay include a permeable skin covering 116, surrounds the support core114 for contacting the wetted surface from which liquid is to beremoved. Vacuum source 120 is coupled to the central cavity of theporous support core 114 for generating air and fluid flow therethrough,extending through the absorbent material layer 112 and the permeableskin 116 to the exterior of the roller member 110. In normal operation,the vacuum source 120 draws liquid carrier that has permeated into theabsorbent material of roller member 110 toward the central cavity of thesupport core 114.

Porous support core 114 may be made from a sintered metal, plastic,ceramic or other rigid material having sufficient rigidity and porosityfor being urged against the liquid developed image while allowingairflow therethrough. In addition, the material is preferably made to beelectroconductive, either by itself or in combination with anotherconductive material, such that the electrical bias provided by supply130 can be applied thereto to produce an electrical field which resultsin a repelling force against the toner particles in the image area.

The conformable microporous absorbent material making up roller 110 ispreferably characterized by an open cell material which may comprise anabsorbent polymeric and/or elastomeric foam material with conductivefiller or dissipative filler incorporated therein. This material has ahardness preferably from 20 to 60 Shore A, and has a thickness of 1.0mils to 500 mils, preferably, about 40 mils to 250 mils. The absorptionmaterial of the microporous roller may be any suitable material,preferably a foam such as one selected from the group consisting ofPolyurethane, Silicone, Fluorocarbon, Polyimide, Melamine, and rubber,such as Permair® (a microporous polyurethane material available fromPorvair Ltd., England), and Textratex® (a microporous semipermeablefluorocarbon membrane available from Tetratec Corp., Pennsylvania).Preferably, the absorbent material is also electroconductive so that theelectric field created by the bias source 130 applied to the core 114 isuniformly distributed along the surface of the roll member 110 and theadjacent image bearing surface. A suitable level of resistivity for theabsorbent material is in the range of 105 to 1011 ohm-cm, and ispreferably in the range of 106 to 109 ohm-cm.

The open cell pores of the absorbent material generally may be less than1,000 microns in diameter, and preferably should be in the range ofabout 5 to about 300 microns, although various applications outside ofthe field of electrostatographic printing may certainly contemplate theuse of pore sizes outside of these limits. Moreover, in the case ofliquid developing material based electrostatographic applications, verysmall pores of one micron or less may be used to absorb liquid carrierfrom an image, resulting in a requirement for increased vacuum pressurenecessary to extract an equivalent amount of liquid as that of a rollerhaving larger size pores. Preferably, the porous absorbent layer issubstantially impervious to toner particles while being pervious toliquid carrier for inhibiting the departure of toner particles from theimage. An exemplary absorbent roller having a rigid porouselectroconductive support core and a conformable microporous roller isdescribed in commonly assigned U.S. Pat. No. 5,481,341, the relevantportions of which are hereby incorporated herein by reference. It isunderstood, however, that various and numerous materials known in theart may be satisfactorily used to meet the strength, porosity andconductivity requirements of the liquid extraction system of the presentinvention. The materials must, of course, be compatible with whateverliquid material is being removed.

In operation, roll member 110 rotates in contact with surface 10 (or 80)to encounter the "wet" image. The absorbent layer 112 of roller 110absorbs excess liquid from the surface of the image through the porousskin covering 116, with the excess liquid permeating into the absorbentlayer via capillary action. Vacuum source 120 is coupled to one end ofthe central cavity defined by core 114, generating negative pressure fordrawing liquid that has permeated into the absorbent layer toward thecentral cavity to transport the liquid to a receptacle or some otherlocation which will allow for either disposal or recirculation of theliquid carrier. Thus, porous roller 110, being continuously dischargedof excess liquid, provides continuous absorption of liquid from theimage on surface 10. This process conditions the image by reducing theliquid content thereof while providing an increase in percent solids inthe developed image, thereby improving the quality of the developedimage.

While the vacuum system 120 assists in drawing liquid carrier throughthe absorbent material of the roll member 110 and into a central cavitythereof, where it may then be removed to a collection location, thevacuum system pressure must be carefully selected so as to remove onlyliquid carrier from the image and so as to not have excessive suctionforce capable of also affecting the toner particles from the image. Ithas been found that vacuum pressures ranging from about 0.5 inches ofwater to greater than 45 inches of water, and preferably within therange of 1.0 to about 15 inches of water have been suitable inelectrostatographic applications. It is noted that capillary action inthe porous roll helps initiate the movement of fluid through the rolland the assistance of the vacuum source 120 provides the continued fluidmotion toward the central cavity thereof.

Problematically, however, notwithstanding the careful selection and/oradjustment of negative air pressure provided by the vacuum source 120,it is not uncommon that loosely attached toner particles as well asother contaminants such as paper debris, dust particles, and the likewill be drawn into the pores of the roller member 110. While some ofthese contaminants will be drawn all the way into the central cavity andremoved from the system without causing any problems, it has been foundthat some contaminants and toner particles may build up in the pores ofthe roller member 110, which, in turn, may result in the loss ofabsorption functionality, and may further result in image degradation.

The present invention provides a solution to the problem described aboveby providing positive air pressure for reversing the flow of liquidcarrier through the roll member 110. The reverse flow purges the rollmember 110 of toner particles and other contaminants built up in thepores thereof. Thus, in accordance with the present invention, thevacuum source 120 also provides positive air pressure to core 114 forpushing liquid carrier and residual contaminants therein away from thecore 114 and out of the absorbent material 112, as well as the permeableskin 116, to purge contaminants therefrom, as will be described infurther detail.

The concept of the present invention may be implemented in various ways.In the simplest approach, the vacuum source 120 can include a first,negative pressure, vacuum generator and a second, positive pressure,vacuum generator. In this embodiment, the positive pressure vacuumgeneratore is periodically energized to generate a flow of liquid awayfrom the central cavity of the roller member, as desired. Preferably,periodic energization of the positive pressure vacuum generator can besystematically initiated so as not to impact the printing process. Forexample, the periodic energization of the positive pressure vacuum canbe implemented between print runs, at machine shut down, or during apreprogrammed preventive maintenance cycle.

In an alternative embodiment, vacuum source 120 may be comprised of abidirectional vacuum system wherein the same vacuum mechanism can beutilized to produce both positive and negative pressure airflow. In thecase of the bi-directional vacuum device, positive pressure airflow ispreferably periodically and systematically generated, as desirable.

In another alternative embodiment of the present invention, asillustrated in FIG. 2, core 114 is provided in the form of a segmentedmember which allows rotation of the absorbent layer 112 and skincovering 116 thereabout. In this embodiment, dual vacuum sources areprovided, with a negative pressure vacuum source 122 being coupled to asegment of core 114 associated with that portion of the roll member 110adjacent image bearing surface 10, while a positive pressure vacuumsource 124 is coupled to a segment of core 114 associated with a portionof the roll member 110 not adjacent the image bearing surface 10. Thisembodiment permits opposing vacuum pressures to be delivered to selectedlocalized areas of the roll member 110 depending on whether that area ofthe roll member is adjacent or not adjacent to the liquid image. In thisembodiment, both the positive and negative pressure vacuum devices 122and 124 may be continuously energized such that the roll member 110 maycontinuously draw liquid and be purged of liquid as the roll member 110rotates.

In review, the present invention provides a system for purgingcontaminants and enhancing the vacuum efficiency in a permeable vacuumassisted roll system used to remove liquid from a wetted surface. Inparticular, the system provides enhanced image conditioning of a liquiddeveloped image being delivered to or on an image bearing surface in aliquid ink based electrostatographic printing machine, particularly animage-on-image type multicolor machine. The image conditioning systemincludes an absorbent contact roll member having a vacuum source coupledthereto adapted to draw fluid through the absorbent roll member and alsoto push fluid out of the roll member to purge contaminants therefrom.

It is, therefore, apparent that there has been provided, in accordancewith the present invention, a system for purging contaminants andenhancing the vacuum efficiency in a centrally evacuated permeableroller system for liquid removal from a wetted surface and particularlyfor conditioning liquid images on an image bearing surface in a liquidink type multicolor electrostatographic printing machine, particularlyan image-on-image type multicolor machine. The method and apparatusdescribed herein fully satisfies the aspects of the inventionhereinbefore set forth. While this invention has been described inconjunction with specific embodiments thereof, it is evident that manyalternatives, modifications and variations will be apparent to thoseskilled in the art. Accordingly, it is intended to embrace all suchalternatives, modifications, and variations as fall within the spiritand broad scope of the appended claims.

We claim:
 1. A system for removing excess liquid from a liquid developedimage having toner particles immersed in a liquid carrier on an imagebearing surface, comprising:an absorbent contact member for contactingthe liquid developed image on the image bearing surface to absorb atleast a portion of the liquid carrier therefrom; a bi-directional vacuumsystem coupled to said absorbent contact member for selectivelygenerating a negative pressure airflow through said absorbant contactmember so as to draw absorbed liquid carrier therethrough and a positivepressure airflow through said absorbant contact member so as to pushabsorbed liquid carrier and residual contaminants from said absorbentcontact member.
 2. The system of claim 1, wherein said absorbent contactmember is a roller member having a first portion adjacent the imagebearing surface and a second portion not adjacent the image bearingsurface, includinga rigid porous core defining a central cavity havingsaid bi-directional vacuum system coupled thereto; and a porousabsorbent material layer surrounding said rigid porous core.
 3. Thesystem of claim 2, further including a permeable skin covering saidporous absorbent material layer.
 4. The system of claim 1, furtherincluding an electrical biasing source coupled to said contact memberfor providing an electrical bias thereto having a polarity similar to apolarity of the toner particles to generate an electric field forelectrostatically repelling and compressing the toner particles towardsthe image bearing surface.
 5. The system of claim 1, wherein saidbi-directional vacuum system includes:a first vacuum generating devicefor generating negative pressure; and a second vacuum generating devicefor generating positive pressure.
 6. The system of claim 5, furtherincluding means for selectively periodically energizing said first andsecond vacuum generating devices.
 7. The system of claim 2, wherein saidbi-directional vacuum system includes:a single vacuum generating devicefor producing a first negative pressure airflow through said porousabsorbent material layer, and a second, negative positive pressureairflow through said porous absorbent material layer.
 8. The system ofclaim 7, further including means for selectively energizing said vacuumgenerating device to periodically produce the negative and positivepressure airflow through said porous absorbent material layer.
 9. Thesystem of claim 2, wherein:said rigid porous core includes a segmentedmember which allows rotation of said porous absorbent material layerthereabout, said segmented member including a first portion associatedwith the portion of said roller member adjacent the image bearingsurface and a second portion associated with the portion of said rollermember not adjacent the image bearing surface; and said bi-directionalvacuum system includes a dual vacuum source having a negative pressurevacuum generator, and a positive pressure vacuum generator; wherein thenegative pressure vacuum generator is coupled to the first portion ofthe segmented member associated with the portion of said roller memberadjacent the image bearing surface, and the positive pressure vacuumgenerator is coupled to the second portion of said roller memberassociated with the portion of said roller member not adjacent the imagebearing surface for permitting opposing vacuum pressures to be deliveredto selected localized areas of said roller member.
 10. A liquiddeveloping material based electrostatographic printing machine includinga system for removing excess liquid from a liquid developed image havingtoner particles immersed in a liquid carrier on an image bearingsurface, comprising:an absorbent contact member for contacting theliquid developed image on the image bearing surface to absorb at least aportion of the liquid carrier therefrom; a bi-directional vacuum systemcoupled to said absorbent contact member for selectively generating anegative pressure airflow through said absorbant contact member so as todraw absorbed liquid carrier therethrough and a positive pressureairflow through said absorbant contact member so as to push absorbedliquid carrier and residual contaminants from said absorbent contactmember.
 11. The system of claim 10, wherein said absorbent contactmember is a roller member having a first portion adjacent the imagebearing surface and a second portion not adjacent the image bearingsurface, includinga rigid porous core defining a central cavity havingsaid bi-directional vacuum system coupled thereto; and a porousabsorbent material layer surrounding said rigid porous core.
 12. Thesystem of claim 11, further including a permeable skin covering saidporous absorbent material layer.
 13. The system of claim 10, furtherincluding an electrical biasing source coupled to said contact memberfor providing an electrical bias thereto having a polarity similar to apolarity of the toner particles to generate an electric field forelectrostatically repelling and compressing the toner particles towardsthe image bearing surface.
 14. The system of claim 10, wherein saidbi-directional vacuum system includes:a first vacuum generating devicefor generating negative pressure; and a second vacuum generating devicefor generating positive pressure.
 15. The system of claim 14, furtherincluding means for selectively periodically energizing said first andsecond vacuum generating devices.
 16. The system of claim 11, whereinsaid bi-directional vacuum system includes:a single vacuum generatingdevice for producing a negative pressure airflow through said porousabsorbent material layer, and a positive pressure airflow through saidporous absorbent material layer.
 17. The system of claim 16, furtherincluding means for selectively energizing said vacuum generating deviceto periodically produce the negative and positive pressure airflowthrough said porous absorbent material layer.
 18. The system of claim11, wherein:said rigid porous core includes a segmented member whichallows rotation of said porous absorbent material layer thereabout, saidsegmented member including a first portion associated with the portionof said roller member adjacent the image bearing surface and a secondportion associated with the portion of said roller member not adjacentthe image bearing surface; and said bi-directional vacuum systemincludes a dual vacuum source having a negative pressure vacuumgenerator, and a positive pressure vacuum generator; wherein thenegative pressure vacuum generator is coupled to the first portion ofthe segmented member associated with the portion of said roller memberadjacent the image bearing surface, and the positive pressure vacuumgenerator is coupled to the second portion of said roller memberassociated with the portion of said roller member not adjacent the imagebearing surface for permitting opposing vacuum pressures to be deliveredto selected localized areas of said roller member.
 19. An improvedvacuum assisted permeable roller system for removal of liquid from awetted surface, comprising:a rigid porous core member and an absorbentcontact layer for contacting the wetted surface to absorb at least aportion of the liquid therefrom; and a bi-directional vacuum systemcoupled to said permeable roller system for selectively generating anegative pressure airflow through said absorbant contact layer so as todraw absorbed liquid therethrough and a positive pressure airflowthrough said absorbant contact layer so as to push absorbed liquid andresidual contaminants from said absorbent contact layer.
 20. Theimproved vacuum assisted permeable roller system of claim 19, whereinsaid bi-directional vacuum system includes:a first vacuum generatingdevice for generating negative pressure; and a second vacuum generatingdevice for generating positive pressure.
 21. The system of claim 20,further including means for selectively periodically energizing saidfirst and second vacuum generating devices.
 22. The system of claim 19,wherein said bi-directional vacuum system includes:a single vacuumgenerating device for producing a negative pressure airflow through saidabsorbent contact layer, and a positive pressure airflow through saidabsorbent contact layer.
 23. The system of claim 22, further includingmeans for selectively energizing said vacuum generating device toperiodically produce the negative and positive pressure airflow throughsaid absorbent contact layer.
 24. The system of claim 19, wherein:saidrigid porous core member includes a segmented member which allowsrotation of said absorbent contact layer thereabout, said segmentedmember including a first portion associated with a portion of saidabsorbent contact layer adjacent the wetted surface and a second portionassociated with a portion of said member absorbent contact layer notadjacent the wetted surface; and said bi-directional vacuum systemincludes a dual vacuum source having a negative pressure vacuumgenerator, and a positive pressure vacuum generator; wherein thenegative pressure vacuum generator is coupled to the first portion ofthe segmented member associated with the portion of said absorbentcontact layer adjacent the wetted surface, and the positive pressurevacuum generator is coupled to the second portion of said absorbentcontact layer associated with a portion of said absorbent contact layernot adjacent the wetted surface for permitting opposing vacuum pressuresto be delivered to selected localized areas of said absorbent contactmember.